The invention relates to soil compacting apparatus in which one or more operational characteristics such as the rotational speed of the exciter, the unbalance, the direction of force or the traveling velocity may be varied and which has measuring means and adjusting means for varying the operational characteristics, which adjusting means may be influenced in accordance with the signal delivered by said measuring means.
Soil compacting apparatus, in particular apparatus in which the soil is compacted by vibrations, such as plate vibrators and rollers with vibrating barrels are frequently provided with systems, known in the prior art, by means of which the kind, magnitude and duration of the effects produced by the apparatus on the soil which is to be compacted may be adjusted either in steps or continuously; for example, such systems may vary the velocity at which the apparatus is driven or pulled over the soil which is to be compacted or they may vary to the magnitude of the centrifugal force exerted by such apparatus. The said force may be altered in compacting apparatus with unbalance excitation by means of the unbalance, the excitation rate being retained, and it may also be varied together with the rotational speed of the exciter; it is also possible to vary the unbalance and rotational speed of the exciter relative to each other so that a new vibrator frequency is obtained with the same vibration intensity. In addition to varying the aforementioned two characteristics it is also possible to vary the principal direction of the centrifugal force of a working part, either by pivoting the exciter or by phase displacement between the rotors in the case of exciters with two or more mass force generators. The phase relationships of the vibrations of soil compacting apparatus with a plurality of working parts may also be varied, for example in a first setting to produce a simultaneous maximum action on the soil or in a second setting to produce an alternating effect.
Experience has shown that the kind of soil compacting apparatus, which may be adjusted in the manner described hereinabove, do not provide optimum compacting results on all soils if the previously mentioned operating parameters are fixedly defined, but that instead it is advantageous for a high vibration frequency to be applied to one soil while a low centrifugal force is more advantageous for another soil and a sliding rather than pressing stress is more advantageous for yet another soil. Manufacturers of dynamic soil compacting apparatus therefore provide adjusting means of the kind mentioned heretofore to provide a wider range of applications for such apparatus and to render them universally usable.
In practice there are however substantial difficulties which militate against the envisaged technical progress being achieved. The first and basic reason is due to the fact that the relationships between the action produced by the compacting apparatus on the soil and the displacement phenomena which occur as the result of such action are substantially unknown: according to the prior art, the user is not yet in a position to optimize the vibrator frequency of the apparatus in accordance with accessible soil properties such as particle distribution and water content based on experience or in terms of a mathematical formula.
A further reason is due to the relationship between the vibration technological characteristics of the soil compactor and a change, for example, of the centrifugal force. Most dynamic soil compacting apparatus operate by so-called "jump" progress, that is to say, the exciter force raises the working parts from the soil in certain phases; the parts then perform a ballistic motion initiated by the exciter force and strike the ground at a moment of time which is defined substantially by the laws of free fall, at which time the exciter force is not necessarily orientated towards the soil. This synchronism between impact pulse and simultaneous exciter force, frequently desirable for intensive compacting, can be disturbed by even slight changes - including increases - of the unbalance or centrifugal force so that the "so-called" jump characteristics of the affected working part which defines compaction may experience fundamental changes which cannot be quantitatively controlled.
Finally, there are also certain properties of the bulk itself which is to be compacted which may prevent the desired success being achieved even if the aforementioned problems are assumed to have been solved. The intrinsic dry bulk density of a dumped material fluctuates, rarely less than 3% and frequently more than 5% and this also applies to local differences of water content. The initial fluctuations are retained almost unchanged after final compactions at Proctor values not substantially in excess of 100% if the dumped material is uniformly worked with a compacting apparatus; the final density, assuming a uniform initial bulk density, is practically proportional to the local water content since this, in the same way as in the Proctor test, has a noticeable effect on the compaction achieved with a defined compacting energy. If steps are to be taken to ensure that minimum dry bulk weight values are obtained for a given compacting problem, these fluctuations must be added to the test value, which, although it amounts to only a few percent, nevertheless results in a substantial increase of the work input.
Proposals have been made according to which the adjustment of suitable machine parts or the variation of their characteristic values is related to measured values which are recorded during the compacting operation. A first apparatus of this kind comprises a seismic acceleration pick-up disposed on a working part, subjected to superimposed loading, and manually operated means for varying the rotational speed of the exciter; it is desirable for the said rotational speed to be maintained at or close to the value at which the acceleration pick-up delivers its maximum signal, that is to say, the system comprising the working part and the soil being approximately at resonance under the effect of the periodic exciter force. The disadvantages of this solution to the problem are not only the basic limitation to the control of superimposed load working parts - resonance conditions do not apply to jump operation either in terms of appearance or by way of concept - but also the fact that co-control of the exciter force through the rotational speed and due to the frequently super-critical damping resulting from friction in the soil it is not possible for the resonance to become sufficiently clearly defined and in these cases there is no adequately significant matching criterion for manual regulation.
It has also been proposed to measure the impact energy of a dynamic soil compacting apparatus component which functions in jump operation for the purpose of controlling the traveling velocity of the apparatus relative to said measurement. A common feature of the proposals is the idea of utilizing the operating characteristics of the compacting apparatus as a controlled condition in terms of process control technology, the compactness produced by the apparatus being the "desired value". Such solutions to the problem suffer from the defect that the relationship between the "desired value" and the appropriate controlled condition is hypothetical because, despite intensive research, it has not been possible to establish a generally valid relationship between the dry bulk density of a soil on the one hand and the vibration characteristics of a dynamic compacting apparatus operated on said soil. Apparatus of this kind therefore merely shift the problem of determining suitable operational parameters of the compacting apparatus, that is to say, defining the relationship between these two magnitudes in a specific, individual case. Although progress is achieved, the problem is not yet solved but its extent is merely limited and expressed in concrete terms.
The object of the present invention is to provide means for varying the operating parameters of soil compacting apparatus during operation based on measurements but in conditions which are free of previous limitations; this includes primarily the process-dependent relationship to the superimposed loading or "jump" operation of the compacting apparatus or its working parts and the condition of validity of the relationships between measured value and "desired value" which must be defined, tested and allowed for independently of the apparatus in question.
The invention is based on the idea to arrange the methods for recording measured values so that on the one hand they become independent of the vibration characteristics of the apparatus or its working parts and on the other hand can be related to soil characteristics, relevant to output, more directly than this is possible according to the prior art.
In this sense it is a further object of the invention to differentiate the solution of the general problem in accordance with different performance features, for example, relative to the compressive strength or shear strength in addition to the compactibility.
According to the basic idea of the invention, the measuring means are constructed as measuring transducer for physical soil characteristics of the soil which is to be compacted or which is to be partially or solely compacted.
According to the invention, the vibration characteristics of the soil compacting apparatus are not utilized as controlled condition as in the prior art but the physical soil characteristics themselves are utilized to function as controlled condition.
The invention may be performed by trailing measuring means being provided which are constructed as measuring transducers for detecting physical soil characteristics after a pass of the soil compacting apparatus. This is not genuine regulation since the soil compactor operation characteristics, influencing the compaction of the soil which is to be freshly compacted, are varied in accordance with the characteristics of the soil which has already been compacted, this change of the operating characteristics of course having no further effect on the characteristics of the soil which is already compacted. Nevertheless, the method may be employed since generally it is possible to assume a degree of constancy of the soil characteristics.
The trailing measuring means may be constructed as measuring transducer for one or more of the following physical soil characteristics after the passage of the compacting apparatus or individual working parts thereof:
(a) compactibility PA1 (b) coefficient of soil reaction PA1 (c) shear strength PA1 (d) continuous vibration impedance PA1 (e) pulse or impact impedance PA1 (f) penetrometric properties of the soil surface PA1 (g) set of the soil surface. PA1 (a) Compactibility PA1 (b) Water content
One or more command signals may be transmissible to the final control means in the manner of command values in process control technology, the signals of the measuring transducers and the command signals being connected in opposition to each other in an adding stage and, where appropriate, being adapted to act through a control amplifier on the final control means.
Finally the object of the invention is to achieve the desired advance, possible, according to the prior art, only by utilizing hypotheses on the relationship between the measured value and the desired value by adopting a solution to this problem of the unknown part of the controlled apparatus.
In this connection the invention is based on the principle that advantageous or optimum adjustment of the operational parameters of dynamic soil compacting apparatus cannot be achieved with means and models of conventional process control technology owing to the special features of the particular art in question. Process control technology is based throughout on a knowledge of the relationship between measured value and final control value, that is to say, the characteristic of the controlled apparatus and only in this way is it possible for the control deviation to form the final control value which will sensibly drive the functional value in terms of magnitude and direction to the reference value. In the present case, the soil to be compacted represents at least part of the controlled apparatus and is therefore variable not only from building site to building site but also within individual compacting areas and furthermore it has a noticeable effect on the operating characteristics of the apparatus and moreover defines its reaction to changes of the final control element, for example, the throttle of the prime mover for controlling the rotational speed.
A further embodiment of the invention therefore provides that supplementary signals of very low frequency may be additively superimposed on the command signals and a transfer signal is formed by a multiplier from the changed command signal and the measuring transducer signal changed thereby through the controller and the controlled part of the apparatus, the said transfer signal being adapted to vary the transfer coefficient of the controller through another multiplier.
The following means may be used for measuring the compactibility of the soil before, after and during the pass of the compacting apparatus:
Radio isotope measurements with gamma rays; in this measuring system a receiver measures the intensity of the reflected radiation which expresses the moist bulk density of the soil by reference to a relationship which must be empirically determined and which is practically independent of the soil. Since it is not necessary for these means to be manually moved it is possible for shieldings to be thicker than those of conventional field probes and accordingly it enables sources to be employed which have activities higher than 20 mC and thus enable the integration periods for the receiver pulses to be reduced. This method may be combined in known manner with corresponding measurement of back scattered thermal neutrons to enable the dry bulk weight to be displayed.
Measurement of the electric soil resistance by means of a four-probe system. The said four-probes are preferably formed by four substantially disc-shaped members with semicircularly radiused edges, electrically insulated from each other and guided on a common shaft. They are rolled over the measuring position with a corresponding slight pressure. The current passing through the outer probes and required to maintain a controlled voltage between the inner probes is a clear measure for the dry bulk weight if the water content is known.
A test ram or a test baulk, bearing hydraulically on soil at a defined pressure, for example 5 kgf/cm.sup.2, may be used as means for measuring the compactibility coefficient of the soil (elastic constant referred to the loaded surface) the amount of set being recorded and stored by a transducer on the baulk guide from the initial contact to approximately 5 seconds after full load is reached. To otain a rapid sequence of such measured values it is possible for a plurality of test baulks of the kind heretofore described to be disposed on the circumference of a hydraulically operated measuring cylinder - individually freely rotatable over corresponding angular ranges.
A plate or baulk, placed on the ground at a pressure of approximately 1 kgf/cm.sup.2 and then retained in its vertical position is suitable as transducer for measuring the shear strength of the compacted soil. The measuring transducer in the more closely defined sense is a dynamometer for defining that force, applied to the said plate by the compacting apparatus or by the tractor, at which the said plate begins to move (in the direction of the force) relative to the adjacent soil surface. The barrel of a roller, rolling with moderate pressure on the soil and driven by an unbalance exciter, may be used for measuring the continuous vibration impedance. In acceleration pick-up with a vertical operating direction defines the accelerations of the measuring roller and therefore also of the soil under the effect of the alternating harmonic force transmitted under the effect of the exciter; the ratio of these two magnitudes is the impedance of the soil.
The pulse or impact impedance is the reciprocal of the Laplace-transformed derivation of the weight function (referred pulse response). The zones of minimum frequency, corresponding to those time intervals from the pulse time at which the deformation velocity becomes zero, that is to say, when the soil begins to swing back, are of significance for a knowledge of the soil characteristics. If the soil is hard-elastic, these periods will be short. If on the other hand the soil characteristics vary from plastic to plastic-flowing, these periods of time will be long to practically infinite. The values may be measured by attaching a velocity pick-up on a drop weight, the said velocity pick-up being adapted to operate an integrating member from the time of impact to the time at which its output signal becomes zero; the measured value is the appropriate final value of said integrator.
Penetrometric soil properties can also be measured by a system incorporating a cylinder which rolls under a certain thrust on the soil, the cylinder barrel having teeth or spikes surmounted upon it which, under the applied thrust, penetrate to a greater or lesser depth into the surface of the soil. The penetration depth is measured by a distance transducer, for example, as the distance between the axis of such a spiked cylinder and a smooth cylinder, guided axially parallel thereto and also rolling on the soil.
It is not usually possible to base the operation of soil compacting apparatus which is to be suitable for any kind of material to be compacted, on a knowledge of the characteristics of the controlled system. It is therefore not possible to predict the sense in which the operating characteristics of the apparatus, for example, the unbalance, must be changed in the event of a deviation of the measured soil characteristics from a set value in order to cause such deviation to disappear. Modulation of the command variable in conjunction with multipliers for automatically changing the control characteristics will be utilized in the above described manner for such apparatus. It is also possible for multi-purpose apparatus to be provided which can be switched to different pre-programmed control characteristics for the purpose of adaptation to different materials if the effect of a change of operating characteristics on the achieved compaction is known. Finally, it is also possible for single-purpose machines to be provided which are intended for use on soils with a uniform or rather similar relationship to one operating characteristic and in which the regulating direction and slope of regulating direction are designed and defined with respect to the purpose of the apparatus. Finally, it is also possible for leading measuring means to be provided, said means being constructed as a measuring transducer in front of the compacting apparatus or the first working member thereof for one or more of the following soil-physical characteristics:
and that the signals of the leading measuring means may be applied to the regulating means in the sense of disturbance-variable feed-forward.